Conventionally, an active matrix liquid crystal display device (Hereinafter referred to as LCD device) which adopts a TFT (Thin Film Transistor) or the like has been widely known. Such an LCD device, as shown in FIG. 11, includes a liquid crystal panel 51 in which liquid crystal 54 is interposed between a TFT-side glass substrate 52 and CF (Color Filter)-side glass substrate 53 so arranged as to face each other. The liquid crystal panel 51 is compartmentalized by scan signal lines and video signal lines, forming liquid crystal cells (pixels) arranged in a matrix manner, and an image is displayed on the liquid crystal panel 51 by controlling an alignment direction of liquid crystal molecules on a cell-by-cell basis for the respective liquid crystal cells.
The alignment direction of the liquid crystal molecules in the liquid crystal cell is controlled by (i) a voltage impressed to a counter electrode formed on a surface of the CF-side glass substrate 53, and (ii) a voltage impressed, by an ON/OFF of the TFT in each of the liquid crystal cells, to a pixel electrode on the TFT-side glass substrate 52.
Generally, in order to ensure reliability of the liquid crystal material, the LCD device is driven by inverting polarity of the voltage being impressed to the liquid crystal of the respective pixels, at a predetermined interval (i.e. an alternate current driving). Examples of such an alternate current driving method for the LCD device are a line-inversion method, a source-inversion method, and a dot-inversion method. In the line-inversion method, image signals are impressed to each liquid crystal cell by inverting the polarity on a line-by-line basis. For example, as shown in FIG. 12, in the line-inversion method, the polarity of the voltage impressed to the liquid cell is inverted by changing, every 1 horizontal (1H) period, (i) the voltage impressed to the counter electrode (solid line) and (ii) the voltage impressed to the liquid crystal cell (dotted line), the voltage corresponding to an image signal.
As described, a state of the liquid crystal while the alternate current driving is carried out is similar to a state of an electrostatic speaker. More specifically, as shown in FIG. 13, in the electrostatic speaker, a conductive thin film is provided between a pair of net-like fixed electrodes. Each of the net-like electrodes respectively receives signals whose phases are opposite to each other, and a sound is produced by impressing a voltage (bias) to the conductive thin film, thus causing the conductive film to vibrate. Similarly, while the liquid crystal is driven by alternate current driving, a pair of electrodes, i.e. counter electrode and pixel electrode, respectively receive a signals whose phases are opposite to each other. The voltage (bias) is impressed to these electrodes.
Accordingly, by driving the LCD device using the line-inverting method, the CF glass substrate 53 vibrates in accordance with the impression of the voltage to the counter electrode, i.e. in accordance with the driving of the counter electrode. Driving frequency of the counter electrode is about 10 kHz in a typical liquid crystal panel for use in typical mobile phones. This causes the CF glass substrate 53 to vibrate at about 10 kHz, while the LCD device is being driven. Since a frequency of the vibration is within an audible frequency band for human being, a user-aggravating sound (noise) is produced while the LCD device is being driven.
For example, in order to reduce the noise generated in the LCD device, Japanese Unexamined Patent Publication No. 8-179285/1996 (Tokukaihei 8-179285; published on Jul. 12, 1996) suggests that the driving frequency of the counter electrode be made higher than an audible frequency band for human being, and that the vibration be attenuated by providing a damping material in the liquid crystal display element.
However, the foregoing method disclosed in Japanese Unexamined Patent Publication, in which the driving frequency of the counter electrode is increased, the driving frequency of the counter electrode is simply increased in a conventional driving method for driving an LCD device, so that the noise is reduced. Simply increasing the driving frequency of the counter electrode causes not only deterioration in operation characteristics of the LC panel, but also an increase in power consumption. Consequently, it becomes difficult to reduce the power consumption in the LCD device.
Further, a provision of the damping material in a liquid crystal display element complicates a configuration of the LCD device. Further, a step for providing the damping material becomes necessary in a process for manufacturing of the LCD device, thus the process for manufacturing the LCD device will become complicated.